1. Field of the Invention
The invention is related to power amplifiers for electric motors and in particular to a power amplifier having an anti-log gain characteristic for activating an electric motor in an automotive electrically assisted power steering mechanism.
2. Prior Art
Power amplifiers capable of controlling the torque output of an electric motor as a function of an input signal are well known in the servo system art. In general the gain characteristics of these power amplifiers are selected to optimize the operation of the servo system as a function of one or more operational parameters. In positioning devices, such as robots or radar antennas, the gain may be a simple function of displacement indicative of distance between the actual and desired positions of the device being moved. Alternatively the gain of the amplifier may be a complex function of the displacement to account for the operational characteristics of the motor, load on the motor, inertial factors, speed of response, positioning accuracy, etc. In most cases, the gain characteristics of the power amplifier are specifically tailored to meet the design objectives of the system.
The present invention is a power amplifier for the electric motor in an electrically assisted power steering mechanism for down sized automotive vehicles. As is known in the art, the primary advantage of electrically or hydraulically assisted steering is when the vehicle is standing or when the driver is executing slow speed maneuvers such as parking. At nominal driving speeds, the requirement for hydraulic or electrical power assist may be eliminated as taught by Turible in U.S. Pat. No. 3,351,152 or reduced or limited as taught by Lear in U.S. Pat. No. 3,885,759, Schimkat in U.S. Pat. No. 3,782,490 or Bayle in U.S. Pat. No. 3,983,953.
The invention is a power amplifier for an electrical assisted power steering mechanism which eliminates the requirement of a vehicle speed input and which eliminates or reduces the authority of the power assistance at nominal and higher vehicle road speeds thereby preserving for the operator what is commonly called "the feel of the road".